The present invention relates to a system for controlling the output power of a motor vehicle such as an automobile, for example.
Road conditions abruptly vary from time to time while a motor vehicle is running, or a motor vehicle often runs on slippery road surfaces with low coefficients of friction, such as snow- or ice-covered roads. In such cases, the drive wheels of the motor vehicle are apt to spin, making it difficult for the driver to control the motor vehicle.
One practical way of getting the motor vehicle under control in such conditions is for the driver to adjust the depression of the accelerator pedal for controlling the output power of the engine so that the drive wheels do not spin. However, even highly skilled drivers find it difficult to make fine accelerator pedal adjustments while driving.
There has been proposed a motor vehicle output power control system for detecting a spinning condition of a drive wheel of a motor vehicle and forcibly lowering the output power of the engine on the motor vehicle irrespective of the depression by the driver of the accelerator pedal if the driven wheel spins. The driver can select a control mode in which the motor vehicle output control mode is in operation to control the output power of the engine, or a normal mode in which the output power of the engine is controlled solely depending on how deeply the accelerator pedal is depressed.
According to known principles of such a motor vehicle output power control system, the rotational speeds of drive and driven wheels are detected with the difference between the detected rotational speeds being regarded as a slip of the drive wheel, and the drive torque produced by the engine is controlled depending on the slip. More specifically, the road conditions are estimated on the basis of a change in the rotation of the driven wheels, and a reference drive torque is established for the engine under the estimated road conditions. The established reference drive torque is then corrected on the basis of the difference between the rotational speeds of the drive and driven wheels so that the drive torque produced by the engine will be equalized to the corrected reference drive torque.
However, since actual road conditions vary from road to road, it would be highly difficult in practice to make accurate estimates of the coefficients of friction of road surfaces with respect to the tires of the wheels. Therefore, it is necessary to employ many corrective factors to establish control conditions. Use of many corrective factors results in a complex control program, which is responsible for a control delay and an increase in the cost of the arithmetic unit of the motor vehicle output power control system.